Sounding reference signals (SRS) are signals that are transmitted by UEs to, e.g., allow the eNodeB to estimate different uplink-channel properties. These estimates may be used for uplink scheduling and link adaptation but also for downlink multiple antenna transmission, especially in case of TDD where the uplink and downlink use the same frequencies. The SRS are defined in FIG. 1 and have time duration of a single OFDM symbol.
SRS can be transmitted in the last symbol of a 1 ms uplink subframe, and for TDD, the SRS can also be transmitted in the special slot, UpPTS. The length of UpPTS can be configured to be one or two symbols. In FIG. 2 an example is illustrated for TDD.
The configuration of SRS symbols, such as SRS bandwidth, SRS frequency domain position, SRS hopping pattern and SRS subframe configuration are set semi-statically as a part of RRC information element.
There are two types of SRS transmission in LTE UL, periodic and aperiodic SRS transmission. Periodic SRS is transmitted at regular time instances as configured by means of RRC signaling. Aperiodic SRS is one shot transmission that is triggered by signaling in PDCCH.
There are in fact two different configurations related to SRS                Cell specific SRS configuration        UE specific configuration.The cell specific configuration indicates what subframes may be used for SRS transmissions within the cell, as illustrated in FIG. 2.The UE specific configuration indicates to the terminal a pattern of subframes (among the subframes reserved for SRS transmission within the cell) and frequency domain resources to be used for SRS transmission of that specific UE. It also includes other parameters that the UE shall use when transmitting the signal, such as frequency domain comb and cyclic shift.        
This means that SRS from different UEs can be multiplexed in the time domain, by using UE-specific configurations such that the SRS of the two UEs are transmitted in different subframes. Furthermore, within the same symbol, sounding reference signals can be multiplexed in the frequency domain. The set of subcarriers is divided into two sets of subcarriers, or combs with the even and odd subcarriers in each such set. Additionally, UEs may have different bandwidths to get additional FDM. (The comb enables frequency domain multiplexing, or FDM, of signals with different bandwidths and also overlapping). Additionally, code division multiplexing can be used. Then different users can use the same time and frequency domain resources by using different shifts of a basic base sequence.
In LTE networks, there are many kinds of downlink heavier traffic, which leads to a larger number of aggregated downlink component carriers (CC) than the number of (aggregated) uplink CCs. For the existing UE categories, the typical carrier aggregation (CA) capable UEs only support one or two uplink CCs while up to 5 CCs can be aggregated in DL in Rel-14 timeframe. A larger number of CCs will be supported in future 3GPP releases.
SRS carrier based switching is aiming to support SRS switching to and between TDD component carrier(s), where the component carriers available for SRS transmission correspond to the component carriers available for carrier aggregation of PDSCH, while the UE has fewer component carriers available for carrier aggregation of PUSCH.
SRS carrier based switching is applicable to at least the following CA scenarios:
                Both TDD-TDD and FDD-TDD CA scenarios are included for SRS carrier based switching.        Both inter-band and intra-band cases and mixtures of these two cases for TDD-TDD and FDD-TDD can be considered for SRS carrier based switching.        
Some of the TDD carriers with DL transmission for the UE will have no UL transmission including SRS, and channel reciprocity cannot be utilized for these carriers. Such situations will become more severe with CA enhancement of up to 32 CCs where a large portion of CCs might be TDD. Allowing fast carrier switching to and between TDD UL carriers can be a solution to allow SRS transmission on these TDD carriers so that corresponding reciprocity benefits can be harvested in DL.
Thus, SRS carrier based switching herein means that during certain time resources the UE does not transmit any signal on one carrier (e.g. F1) while it transmits uplink reference signals (e.g. SRS) on another carrier (e.g. F2). To perform SRS switching, the UE uses the radio circuitry (e.g. transmitter chain) of one carrier to transmit SRS on a cell of another carrier. This operation may cause interruption on one or more cells serving the UE. As an example, F1 and F2 can be PCell carrier and SCell carrier respectively, or both of them can be SCells carriers.
In E-UTRAN the serving cell can request the UE to acquire the cell global identifier (CGI) of a cell, which uniquely identifies the cell. To acquire the CGI of the cell, the UE must read at least part of the system information (SI) including master information block (MIB) and the relevant system information block (SIB) of that cell. The reading of SI for the acquisition of CGI is carried out during measurement gaps which are autonomously created by the UE i.e. gaps that are not configured by the network node rather left for the UE to create. The CGI or SI or ECGI is also considered to be a UE measurement, which the UE may also report to the network node.
In LTE the UE reads the MIB and SIB1 of the target cell E-UTRAN cell to acquire its CGI (aka ECGI when the target cell is E-UTRAN intra- or inter-frequency).
In LTE the MIB includes a limited number of most essential and most frequently transmitted parameters that are needed to acquire other information from the cell, and is transmitted on BCH. In particular the following information is currently included in MIB: DL bandwidth, PHICH configuration, and system frame number (SFN).
The MIB is transmitted periodically with a periodicity of 40 ms and repetitions made within 40 ms. The first transmission of the MIB is scheduled in subframe #0 of radio frames for which the SFN mod 4=0, and repetitions are scheduled in subframe #0 of all other radio frames.
In LTE the SIB1 contains at least the following information, PLMN identity, cell identity, CSG identity and indication, frequency band indicator, SI-window length, scheduling info for other SIBs etc.
The LTE SIB1, as well as other SIB messages, is transmitted on a physical channel, PDSCH. The SIB1 is transmitted with a periodicity of 80 ms and repetitions made within 80 ms. The first transmission of SystemInformationBlockType1 is scheduled in subframe #5 of radio frames for which the SFN mod 8=0, and repetitions are scheduled in subframe #5 of all other radio frames for which SFN mod 2=0.
The UE receives request from the network node to acquire CGI of a target cell as indicated by its PCI. The UE receives measurement configuration or an assistance data/information, which is a message or an information element (IE) sent by the network node (e.g. serving eNode B, positioning node etc) to configure UE to perform the requested measurements. The UE therefore first synchronizes to the target cell and acquire PCI of the target cell. The UE then creates autonomous gaps for acquiring automatic gain control (AGC), for reading initial transmissions and repetitions of MIB and SIB1.
The SRS carrier based switching causes interruptions in one or more serving cells of the UE. The interruption may affect a measurement procedure during which the UE is acquiring system information (SI) of a target cell. The interruptions may on one hand result in the UE prematurely aborting the SI acquisition procedure. This will degrade the mobility performance and/or Self-Organizing Networks (SON) operation which relies on CGI of the target cell. Another consequence is that the UE may create more autonomous gaps leading to more interruption for the serving cell. This will degrade the serving cell reception and/or transmission performance for the UE configured to acquire the SI of the target cell.
The approaches described in the Background section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in the Background section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in the Background section.